36 



NERVOUS SYSTEM OF VERTEBRATES. 



from which the sensory nerves are derived is taking on definite 

 form in close relation with the neural tube. The accompanying 

 figures illustrate the development of the neural tube and the 

 ganglia of the sensory nerves in amphibia. As seen in Figure 13 



Fig. 14. — Transverse section of the neural tube of Amblystoma^puitctatum just 

 after closing. The cells which will form the neural crest are more^darkly shaded. 

 Mitotic figures are seen at all levels in the wall of the tube. 



B, the neural plate consists of columnar cells, while the neural 

 folds which bound it consist of two or more layers of somewhat 

 cubical cells irregularly arranged. As the edges of the neural 

 plate close up to form the neural tube (Fig. 14), the neural folds 



Fig. 15. — Transverse section through neural tube, neural crest and ectoderm of 

 AmUystoma punctatum at a later stage than that shown in Fig. 14. 



form a sort of bridge connecting the tube with the ectoderm. Now 

 the cells which originally formed the folds separate from the 

 ectoderm and extend laterally on the surface of the neural tube, 

 between it and the ectoderm (Fig. 15). Some of these cells remain 



